Exhaust gas treatment method and apparatus

ABSTRACT

Exhaust gas treatment method and apparatus extract heat from an exhaust gas by operating in a water-condensing mode which allows more heat to be recovered, removes particulate matter and condensed acid from the exhaust gas, and washes heat exchange surfaces to keep them clean and wet to improve heat transfer. Systems for heating water, air, and both water and air are disclosed. Methods of constructing and assembling improved heat exchangers are disclosed.

This application is a continuation-in-part of my copending applicationSer. No. 81,789 filed Oct. 4, 1979 and Ser. No. 252,297 filed Apr. 9,1981 both abandoned.

My invention relates to exhaust gas treatment method and apparatus, andmore particularly, to improved method and apparatus useful not only forrecovering large amounts of heat from various industrial exhaust gases,but also for simultaneously removing substantial amounts of particulatematter and corrosive products of combustion from such exhaust gases,thereby to reduce air pollution from stack emissions. The invention isparticularly directed toward such treatment of sulfur-containing exhaustgases, such as those typically produced by burning oil or coal infurnaces, though it will become apparent that the invention will beuseful in a wide variety of other applications. A primary object of theinvention is to provide method and apparatus which are useful forrecovering a substantially larger percentage of the heat contained in anexhaust gas than that recovered in typical prior systems, which has veryimportant economic implications, due to the high costs of fuels. Anotherimportant object of the invention is to provide method and apparatuswhich are useful for removing substantial amounts of particulate matterand corrosive products of combustion from exhaust gases, therebydecreasing pollution. Natural gas, #2 fuel oil, #6 fuel oil, and coal,generally ranked in that order, produce flue gases containing increasingamounts of sulfur dioxide and sulfur trioxide, and particulate matter,such as soot and silica products. One object of the invention is toprovide method and apparatus which is useful in connection with fluegases produced by any of those fuels.

In many applications it is desirable that waste heat be used to preheata liquid, such as boiler make-up water, or industrial process water asexamples, while in many other applications it may be preferred thatwaste heat be used to preheat a gas, such as air, and in someapplications to heat both a liquid and a gas. Another object of theinvention is to provide a method which lends itself to preheating ofeither a liquid or a gas or both a liquid and a gas, and to provideapparatuses which preheat liquid or a gas or both a liquid and a gas.

A very important object of the present invention is to provide methodand apparatus which is rugged and reliable, and useful over long periodsof time with minimum attention, and minimum requirements for "downtime"for cleaning or repair.

Another more specific object of the invention is to provide a heatexchanger which functions as a self-cleaning gas scrubber as well asrecovering increased amounts of heat from an exhaust gas.

It long has been known that the thermal efficiency of a plant or processcan be increased by recovering some of the heat energy contained in theexhaust gas from a boiler furnace or the like. Flue gas commonly isdirected through boiler economizers to preheat boiler feedwater, andcommonly directed through air preheaters to preheat furnace combustionair, in each case providing some increase in thermal efficiency. Theamount of heat which it has been possible to recover from flue gasordinarily has been quite limited, due to serious corrosion problemswhich otherwise result. Combustion of oil, coal or natural gas producesflue gas having substantial moisture, sulfur dioxide, sulfur trioxide,and particulate matter in the cases of oil and coal. If a heat exchangerintended to recover heat from flue gas condenses appreciable amounts ofsulfur trioxide, sulfuric acid is formed, resulting in severe corrosion.The condensed sulfur product can readily ruin usual economizers and airpreheaters, and the exhaust stacks associated with them. Thus prior artsystems intended to recover heat from flue gas traditionally have beenoperated with flue gas temperatures scrupulously maintained high enoughto avoid condensation of sulfur products.

The temperature at and below which condensation will occur for a fluegas not containing any sulfur oxides, i.e., the dew point due to watervapor only, is usually within the range of 100° F. to 130° F., dependingon the partial pressure of water vapor. But the presence of sulfurtrioxide even in small amounts, such as 5 to 100 parts per million,drastically increases the temperature at which condensation will occur,far above that for water vapor only. For example, 50 to 100 parts permillion of SO₃ may raise the dew point temperature to values such as250° F. to 280° F., respectively. Thus it has been usual practice tomake absolutely certain that flue gas is not cooled below a temperatureof the order of 300° F., in order to avoid condensation and corrosion.Such operation inherently results in an undesirably small portion of thesensible heat energy being extracted from the flue gas, and inabsolutely no recovery of any latent heat energy contained in the fluegas. One concept of the present invention is to provide method andapparatus for recovering heat from a potentially corrosive exhaust gas,such as flue gas, in a manner directly contrary to prior art practices,using a heat exchanger which continuously operates in a "watercondensing" mode, allowing substantial amounts of latent heat, as wellas more sensible heat, to be recovered from the exhaust gas. The term"water condensing" is meant to mean that the temperature of a largepercentage (and ideally all) of the exhaust gas is lowered not onlybelow the sulfuric acid condensation or saturation temperature, but evenbelow the saturation temperature of water at the applicable pressure,i.e. below the dew point, e.g. 120° F., for water vapor only. In atypical operation of the invention where absolute pressure of the fluegas within a heat exchanger is of the order of 0.2 to 5 inches of water,the temperature of large portions of the flue gas is lowered at leastbelow 120° F. to a temperature of say 75° F. to 100° F., by passing theflue gas through a heat exchanger scrubber unit. The unit continuouslycondenses a large amount of water from the flue gas, as well ascondensing sulfuric acid. Parts within the heat exchanger-scrubber unitwhich would otherwise be exposed to the corrosive condensate areappropriately lined or coated with corrosion-resistant materials, e.g. afluoroplastic such as "Teflon" (trademark of E. I. duPont de Nemours &Co., Inc.), to prevent corrosion.

When prior art waste heat recovery systems have been operated with fluegas temperatures (e.g. 250° F.) too near the SO₃ condensationtemperature, whether by accident, or during startup, or in attempts toimprove system thermal efficiency, the occasional condensation of SO₃tends to produce very strong or concentrated sulfuric acid. The sulfuricacid, is extremely corrosive and can rapidly destroy an ordinary heatexchanger. The volume of sulfuric acid which can be condensed is small,but sufficient to keep heat exchanger surfaces slightly moist. However,if one not only ignores prior art practice, but operates in directcontradiction thereto, and further lowers the flue gas temperature to alevel markedly below the SO₃ condensation temperature, to operate in thewater condensing mode of the invention, the production of large amountsof water tends to substantially dilute the condensed sulfuric acid,making the resultant overall condensate much less corrosive, and lesslikely to damage system parts. Thus operation in the water condensingmode of the invention has an important tendency to lessen corrosion ofsystem parts, as well as allowing large amounts of latent heat energy tobe recovered. Such dilution of the sulfuric acid does not whollyeliminate corrosion, however, so that it remains necessary toappropriately line or coat various surfaces within the heatexchanger-scrubber unit with protective materials.

I have discovered that if I pass flue gas through heat exchangerapparatus which is operating in the water condensing mode, not only canmuch more heat energy be extracted from the flue gas, and not only canthe condensate be made less corrosive, but in addition, large amounts ofparticulate matter and SO₃ simultaneously can be removed from the fluegas, thereby considerably reducing air pollution. I have observed thatif flue gas is cooled slightly below the sulfuric acid condensationtemperature, and if the flue gas contains a substantial amount ofparticulate matter, a soggy mass of sulfuric acid combined withparticulate matter often will rapidly build up on heat exchangesurfaces, and indeed the build-up can clog the heat exchanger in amatter of a few hours. But if the system is operated in the watercondensing mode in accordance with the present invention, the productionof copious amounts of water continuously washes away sulfuric acid andparticulate matter, preventing buildup of the soggy mass. In simpleterms, the water condensing mode of operation forms a "rain" within theheat exchanger. The "rain" not only entraps particles in the flue gas asit falls, but it also washes away, down to a drain, particles which havelightly stuck to the wetted surfaces. Thus advantages akin to those ofgas scrubbing are obtained within a need for the continuous supply ofwater required by most gas scrubbers, and with no need for moving parts.

Yet, even in addition to recovering large amounts of latent heat energy,greatly diluting and washing away sulfuric acid to minimize corrosionproblems, and removing substantial amounts of particulate matter fromexhaust gases such as flue gas, operation in the "water condensing" modealso enhances the heat transfer coefficient of heat exchanger units usedto practice the invention. Heavy condensation made to occur near the topof the heat exchanger unit runs or rains downwardly, maintaining heatexchange surfaces wetted in lower portions of the heat exchanger, eventhough condensation otherwise is not occurring on those surfaces.

Heat transfer is improved for several separate but related reasons. Heattransfer from the gas occurs better if a surface is wet from drop-wisecondensation. Use of a fluoroplastic such as "Teflon" promotes dropwisecondensation. Further, the constant rain within the heat exchanger keepsthe tube surfaces clean, preventing the buildup of deposits which woulddecrease heat transfer. Thus another object of the invention is toprovide improved heat recovery apparatus having improved heat transfer.

While I have found various forms of fluoroplastics to provide veryeffective corrosion protection, and to have hydrophobic characteristicswhich cooperate with water vapor condensation to keep a water-condensingheat exchanger clean, currently available forms of thosecorrosion-protection materials have deformation, melting or destructiontemperatures far below the temperatures of some flue gases from which itis desirable to extract waste heat. In accordance with another aspect ofthe present invention. I propose to operate a water-condensingheat-exchanger in cooperation with another heat exchanger ofconventional type, which may be severely damaged if condensation takesplace in it. The conventional heat exchanger can initially cool anexhaust gas such as flue gas down to a temperature which is low enoughthat it will not damage the corrosion-protection linings of thewater-vapor condensing heat exchanger, yet high enough that sulfurtrioxide cannot condense in the conventional heat exchanger so as todamage it. Thus some added objects of the invention are to provideimproved heat recovery systems which are useful with exhaust gaseshaving a wide range of temperatures.

Some added objects of the invention are to provide heat exchangermodules suitable for use in the mentioned water-condensing mode whichcan be readily combined as needed to suit a wide variety of differentflow rate, temperature and heat transfer requirements. Another object ofthe invention is to provide a satisfactory method of constructing andassembling a water-condensing heat exchanger system.

An important further object of the invention is to provide improvedcondensing heat exchanger systems having improved heat transfercoefficients, so that increased amounts of heat can be recovered perunit area of heat transfer surface. More specifically, one object of theinvention is to provide condensing heat exchangers having markedlyimproved heat transfer by reason of the exhaust gas being forcedvertically downwardly, between and around horizontally-extendingcylindrical tubes which carry the fluid (water or air) being heated,with condensation of water vapor from the exhaust gas occurring onlyadjacent a lowermost group of the tubes. Another object of the inventionis to provide a condensing heat exchanger having such improved heattransfer in which effective removal of particulates also occurs.

Another object of the invention is to provide a condensing heatexchanger in which exhaust gas flow proceeds substantially horizontally,which enables one to provide heat exchangers having modest heights.

Another object of the invention is to provide an improved exhaust gasscrubbing system requiring less water.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combination of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

For a fuller understanding of the nature and objects of the inventionreference should be had to the following detailed description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a diagram of a heat exchanger system useful in understandingsome basic principles of the present invention.

FIG. 2 is a set of graphs useful in understanding principles of onemethod of the present invention.

FIG. 3a is a schematic diagram of one form of water heating systemaccording to the invention.

FIG. 3b is a diagram illustrating use of the invention in connectionwith a direct-fired paper dryer to save heat energy and removeparticulate matter from the exhaust gas from the paper dryer.

FIGS. 3c and 3d each is a diagram illustrating a use of thewater-condensing heat exchanger of the present invention together with aprior art air preheater.

FIG. 4a is a plan view of an exemplary heat exchange module inaccordance with the invention.

FIG. 4b is a partial cross-section elevational view taken at lines4b--4b in FIG. 4a.

FIG. 4c is a diagram useful for understanding the heat exchange tubespacing in a preferred embodiment of the invention.

FIG. 4d is a diagram illustrating one form of water manifolding whichmay be used in connection with the invention.

FIGS. 5a and 5b are front elevation and side elevation views of anexemplary air heat form of water-vapor condensing heat exchanger.

FIG. 6 illustrates an exemplary heat recovery system utilizing a heatexchanger which heats both air and water from boiler flue gas.

FIGS. 7a and 7b are partial cross-section views useful for understandinga method of assembly according to the present invention and the natureof tube-to-tube sheet seals provided by use of such a method.

FIGS. 8a and 8b are side view and end view diagrams of a gas downflowversion of the condensing heat exchanger of the invention, and

FIG. 8c is a side view of a horizontal gas flow version of theinvention.

FIG. 9 is a diagram helpful in understanding the function of theimproved downflow model heat exchanger of FIGS. 8a and 8b.

FIG. 10 is a set of graphs useful in understanding principles of the gasdownflow heat exchanger of FIGS. 8a and 8b.

FIG. 11a is a partial plan view illustrating a heat exchanger moduleincorporating an intermediate tube support.

FIG. 11b is a sectional view of an intermediate tube support taken atlines 11b--11b in FIG. 11d.

FIG. 11c is a sectional view illustrating details of the intermediatetube support.

FIG. 11d is a side view illustrating the intermediate tube supportsheet.

FIG. 12a is a diagram showing one alternative flue gas flow controlsystem for use with a single boiler.

FIG. 12b is a further alternative flue gas flow control system for amultiple boiler system.

Some major principles of the present invention can be best understood byinitial reference to FIGS. 1 and 2. In FIG. 1 duct 10 conducts hotexhaust gas, such as flue gas drawn from a boiler stack (not shown) byblower B, to a bottom plenum or chamber 11. The flue gas passes upwardlythrough one or more heat exchange units, such as the four unitsindicated at 12, 13, 14 and 15, thence into an upper plenum or chamber16 and out a stack 17. In typical operations the flue gas velocity isarranged to be 10-40 feet per second within the heat exchange units, anda gas pressure drop of the order of one and a half to two inches ofwater is arranged to occur across the heat exchange units. A fluid to beheated, which typically will be water or air, is shown introduced intothe uppermost heat exchange unit at 20, understood to flow downwardlythrough successive ones of the heat exchange units, and to exit at 21.For simplicity of explanation it initially will be assumed that water isto be heated, and that the hot exhaust gas is flue gas from a boiler.

It will be apparent that the flue gas will be cooled to some extent asit travels upwardly through the unit, and that the water will be heatedto some extent as it travels downwardly through the unit. To facilitateexplanation, the range of elevations within which significant gascooling and water heating occur is shown divided into four zones Z1 toZ4. The four zones are shown for simplicity of explanation ascorresponding to the vertical ranges of the four heat recovery units.

FIG. 2 illustrates the variations of flue gas and water temperatures intypical practice of the invention to heat water, with the temperaturesplotted against vertical elevation. Thus the temperature of flue gasfalls from an assumed input temperature G₁ of 500° F. at the bottom ofthe heat exchanger system to an assumed output temperature G₀ of 90° F.at the top of the heat exchanger system as the flue gas travels upwardlythrough the heat exchanger, as indicated by curve G. The gas temperatureplot in FIG. 2 should be understood to be approximate, and in general todepict for any elevation the lowest temperature to which substantialportions of the gas are lowered at that elevation. At any elevationabove the lowermost row of tubes there are temperature gradients, ofcourse, and the average temperature, if averaged over the entirecross-sectional area of the heat exchanger at a given elevation, will beabove that plotted as curve G. Viewed in another way, at a givenelevation, such as that bounding zones Z2 and Z3, some portions of theflue gas, such as portions near or on a tube may have the temperature G₂at which sulfuric acid is forming, while other portions of the gas atthe same elevation but at greater distances from any tube may be hotterand not yet experiencing condensation. Simultaneously, water assumed tohave an input temperature W₁ of 55° F. at the top of the heat exchangeris heated to an output temperature W₀ of 180° F. by the time it reachesthe bottom of the heat exchanger, as indicated by curve W. The ordinatescale in FIG. 1 is shown divided into the four zones Z1 to Z4. Avertical dashed line F at 250° F. indicates a typical temperature G₂ atwhich sulfuric acid forms in typical flue gas obtained from burning No.6 fuel oil. The prior art has taught that flue gas temperature alwaysshould be maintained amply above that level in order to avoid productionof any sulfuric acid.

Following the flue gas temperature curve G for upward flue gas travel,it will be seen that the flue gas temperature drops from 500° F. at G₁to 250° F. at G₂ as the gas passes through the two lower zones, Z1 andZ2. During that portion of its upward travel very little condensation isoccurring from the gas, but sensible heat from the flue gas is beingtransferred to heat the water. More precisely, most of the gas in zonesZ1 and Z2 is still too hot for sulfuric acid to condense, but someamounts of the gas in zones Z1 and Z2 will condense while in thosezones, because condensate falling through those zones from above willcool some amounts of gas in those zones to the condensation temperature.Even though little or no condensation from the gas is occurring in lowerzones Z1 and Z2, the gas-side surfaces of heat exchanger tubes in thoselower zones will continuously remain wet, by reason of copiouscondensation falling from above. The wet condition of those tubes causesparticulate matter to tend to lightly and temporarily stick to them, butcondensate falling from above acts to continually wash them, washingsulfuric acid and particulate matter down to the drain (D, FIG. 1) atthe bottom of the heat exchanger unit.

As the flue gas is cooled below 250° F., a typical dew point for sulfurtrioxide, sulfuric acid forms in zone Z3. If zone Z3 were the uppermostzone in the system, the condensed sulfuric acid would slightly wet heatexchange surfaces in zone Z3, and particulate matter would build uponthose slightly wetted surfaces. The sulfur trioxide condenses to formsulfuric acid in the assumed example, as gas travel through zone Z3cools the gas substantially below the acid dew point. At some levelwithin zone Z3 most or all of the sulfuric acid which will be condensed,will have condensed. Just above that level there is believed to be arange of elevations, indicated generally by bracket R in FIG. 2, inwhich one might deem little or no condensation to be occurring, the gastemperature being low enough that condensation of sulfuric acid has beenlargely completed, but being too high for water vapor to condense insubstantial quantities. However, as the gas is further cooled duringfurther upward travel through zone Z1, condensation of water vaporoccurs in an increasingly copious manner. Thus all of the tubes in theheat exchange units are continuously wet, which not only increases heattransfer, but which, at the mentioned gas flow rate, also causesparticulate matter to tend to temporarily and lightly stick to thetubes, removing large amounts of particulate matter from the gas. Thewater caused by continuous condensation acts to continuously wash thesurfaces of the heat exchange tubes, washing particulate matter andcondensed sulfuric acid down to the drain at the bottom of the system.The tubes within the heat exchange units are arranged in successive rowswhich are horizontally staggered relative to each other, so that a dropof condensate falling from one tube tends to splash on a tube below. Adouble form of cleaning of the flue gas occurs, from the combinedscrubbing of the gas as it passes through the falling rain ofcondensate, plus the tendency of the particulate matter to temporarilyand lightly adhere to the wetted tubes until condensate washes it away.The thin coverings of fluoroplastic on the heat exchange tubes not onlyprevent corrosion of those tubes while allowing good heat transfer, butbecause those coverings are hydrophobic, the falling condensate isreadily able to wash away particulate matter from the tubes. Testsrecently conducted by Brookhaven National Laboratory indicate thatapproximately 70%±5% of the total suspended particulates and 20% to 25%of the SO₃ in flue gas from No. 6 fuel oil can be removed. In a typicalapplication of the invention, if say 9660 pounds per hour of flue gasfrom No. 6 oil are passed through the water condensing heat exchanger,approximately 250 pounds per hour of water mixed with sulfuric acid andparticulate matter will be drained from the bottom of the unit. Theparticulate matter in the mixture has been about 3% by weight in testsmade to date.

The amount of sulfur trioxide in typical flue gas is measured in partsper million, so that the amount of sulfuric acid which even completecondensation of sulfur trioxide could produce is much, much less thanthe amount of water which can readily be condensed if the flue gas iscooled sufficiently.

The precise temperature at which water vapor in flue gas will condensevaries to some extent in different flue gas mixtures from 100° F. to130° F., but it is typically 120° F. (49° C.). Thus in accordance withthe present invention, water (or air) at or below that temperature, andpreferably well below that temperature, is maintained in a group oftubes near the top of the heat exchanger unit, insuring that largeamounts of water vapor condense. In FIG. 2 the water temperature isassumed to rise from 55° F. to about 75° F. as the water passesdownwardly through zone Z4. Thus substantial tube volume at the upperportion of the heat exchanger system in FIGS. 1 and 2 contains waterbelow 120° F.

It may be noted that in the example of FIG. 2, the water temperaturereaches the water vapor dew point (120° F.) at substantially the sameelevational level where the flue gas temperature reaches the sulfuricacid dew point (250° F.), and further, that the level occurssubstantially at the vertical mid-level of the heat exchanger. Thoseprecise relationships are by no means necessary.

In FIG. 2 curve D is a plot of the difference between flue gastemperature and water temperature versus elevational level in the heatexchanger unit. The temperature difference at a given level has animportant bearing, of course, on the amount of heat transfer whichoccurs at that level. It is important to note that near the bottomportions of the heat exchanger unit, in zones Z1 and Z2, there ismaximum difference between gas temperature and water temperature andhence a maximum potential for effective heat transfer.

It may be noted that in most systems constructed in accordance with theinvention, the flue gas exit temperature, measured in chamber 16 orstack 17 in FIG. 1, for example, will be less than the flue gas watervapor dew point temperature of say 120° F., but in some systems the exittemperature at such a location may exceed that dew point to some extent,without departing from the invention. If the heat exchanger tube andhousing geometry allows some quantum of flue gas to pass through theunit with only modest cooling, that quantum will mix in chamber 16 withflue gas which has been cooled sufficiently to condense large amounts ofwater, and tend to raise the average or mixture temperature in chamber16, in the same manner that by-passing some flue gas around the heatexchanger and admitting it to chamber 16 would raise the averagetemperature in that chamber.

From the above it will be seen that the method of the inventioncomprises simultaneous recovery of both sensible and latent heat from ahot exhaust gas containing water vapor, a condensable corrosiveconstituent (sulfur trioxide) and particulate matter, and removal ofsubstantial amounts of the particulate matter and condensed corrosiveconstituent from the gas, by passing the gas through a gas passage of aheat exchanger, simultaneously passing a fluid cooler than the exhaustgas through a second passage of the heat exchanger in heat exchangerelationship with the gas passage, with the flow rates of the gas fluidarranged in relation to the heat transfer characteristics of the heatexchanger so that continuous condensation of water vapor and thecorrosive constituent occurs, providing falling droplets which captureand wash away portions of the particulate matter and condensed corrosiveconstituent.

While prior art economizers and air preheaters require hot input wateror air, the invention advantageously can use cold water or air, andindeed, efficiency increases the colder the input fluid is, with morecondensation occurring and more latent heat being extracted from theexhaust gas. The cooler the water at inlet 20, the lower the exittemperature of the exhaust gas will be, the more latent heat will berecovered, the more water vapor will be condensed from the exhaust gas,and the more effective particulate and SO₃ removal will be, for givenflow rates. Tests to date have indicated that with a gas velocity of theorder of 30-40 feet per second, provision of sufficient heat exchangesurface area will enable one to lower the temperature of the exitingflue gas to within about 8° F. of the input temperature of the fluidbeing heated, be it water or air. For example, with water being suppliedto the heat exchanger at 55° F., it has been possible to obtain flue gasexit temperatures of 63° F. In most applications of the invention itwill not be deemed necessary to lower the flue gas temperature to thatextent, and the amount of heat exchange surface will be selected so thatwith the desired flow rates and fluid temperature the flue gastemperature will be lowered to within the range of 80° F.-100° F.Typical flue gases contain 5% to 12% water vapor, depending upon thetype of fuel, so if 10,000 pounds of flue gas are passed to a heatexchanger in an hour, that represents 500-1200 pounds of water per hour.By operating in the water-condensing mode, several hundreds of pounds ofwater will condense per hour.

The advantageous effect which very substantial water condensation has onheat transfer has been illustrated by operating one form of theinvention under two sets of operating conditions. The unit was firstoperated with flue gas produced by burning No. 2 fuel oil, withparticular inlet and outlet temperatures and flow rates of flue gas andwater. The heat recovered was measured to be approximately 1,000,000 Btuper hour, and condensate flowed from the unit at approximately one-halfgallon per minute. Under such circumstances the latent heat amounted toapproximately 24% of the total heat being recovered. Then later, fluegas produced by burning natural gas was used. The natural gas can be andwas burned with less excess air, and due to the greater hydrogen contentof the natural gas, the flue gas contained a greater amount of moisture.At the same gas and water flow rates as had been used with fuel oiloperation, the amount of condensate which flowed from the unit wasessentially doubled, to approximately a full gallon per minute. However,the water outlet temperature increased and the gas exit temperaturedecreased, and the heat recovered had increased approximately 20%, to1,200,000 Btu per hour. Under these circumstances the latent heatamounted to approximately 40% of the total heat being recovered.

In the application depicted in FIG. 3a flue gas is drawn from theconventional stack 30 of boiler 31 past one or more damper valves by aninduced draft blower B driven by blower motor BM, to supply the flue gasto the bottom plenum of the heat exchange units HX, and cooled flue gasexits via fiberglass hood 16 and fiberglass stack 17 to atmosphere. Theuse of fiberglass stack to resist corrosion is not per se new. Coolwater from the bottom of hot water storage tank ST, from a cold watersupply line SL and/or from a water main WM, is pumped through the heatexchange unit HX by circulator pump CP, and heated water from the heatexchanger is pumped into storage tank ST near its top. Hot water isdrawn from the top of the storage tank via line HW for any of a varietyof uses. Make-up water for boiler 31 can be supplied from tank ST, ofcourse, or directly from unit HX.

If insufficient hot water is drawn via line HW, the contents of storagetank ST can rise in temperature enough that insufficient flue gascooling begins to occur in the heat exchange unit, tending to endangerthe fluoroplastic corrosion-protection coverings and liners in thatunit. A conventional thermal sensor TS senses outgoing flue gastemperature and operates a conventional positioner PV1, which closesdamper valve DV1 to decrease or terminate passage of hot flue gas to theheat exchanger. In FIG. 3a a second positioner PV2 also responsive tothermal sensor TS is shown connected to operate damper valve DV2, whichcan open when stack 17 temperature climbs too high, to mix cool ambientair with the flue gas, thereby to prevent temperature within the heatexchanger from exceeding a value (e.g. 550° F.) deemed dangerous for thecorrosion-protective coatings. In many applications only one or theother of the two described heat limit protecting means will be deemedsufficient. As will become clear below, preventing a temperature risewhich would damage the corrosion-protection coatings can instead be doneby using a conventional non-condensing heat exchanger to cool theexhaust gas to a safe operating temperature before the exhaust gas ispassed through the water-condensing heat exchanger.

In FIG. 3a a spray manifold SP carrying one or a plurality of nozzles isoperative to spray water down through heat exchanger HX when valve V isopened, to wash away any deposits which might have built up on heatexchanger tubes. Because operation in the water condensing modeordinarily functions to keep the tubes clean, operation of such a spraycan be quite infrequent, and in some applications of the inventionprovision of such a spray means may be deemed wholly unnecessary. Incertain applications, such as where particulate removal is deemedparticularly important, valve V can be opened to permit a continuousspray, to augment the "rain" caused by water vapor condensation.

Practice of the invention may be carried out in order to heat air ratherthan water, utilizing heat exchanger apparatus which is extremelysimilar to that previously described for water heating. While use ofcopper tubes is preferred for water heating, aluminum tubes arepreferred for air heating. In either case corrosion-protection coatingssuch as a fluoroplastic, e.g. "Teflon" are used.

Practice of the invention is not restricted to treatment of boiler orfurnace flue gases, but readily applicable to a variety of other hotexhaust gases. In the papermaking industry it is common to providedirect-fired dryers in which large amounts of ambient air are heated,ordinarily by burning No. 2 fuel oil, and applied by large blowers todry webs of paper. The heated exhaust air which has passed through thepaper web contains substantial amounts of paper particles, as well asthe usual constituents of flue gas, with more than usual amounts ofmoisture. Only a limited amount of the heated air can be re-circulated,since its humidity must be kept low enough to effect drying, sosubstantial amounts of makeup air are required. Heating outside orambient air to the temperature desired for paper drying requires a largeamount of fuel. Prior art heat recovery techniques have been clearlyunsuitable in such an application. Ambient air is always cool enoughthat passing such air through a usual air preheater would result incondensation of sulfuric acid from the exhaust gas, causing corrosion,and in the presence of condensation the paper particles rapidly stick toand build up on moist surfaces within the heat exchanger, clogging it. Ido not believe any technique for recovering heat from a dryer has beensuccessful. In accordance with the present invention, as illustrated inFIG. 3b, hot (e.g. 540° F.) paper-laden exhaust air from a conventionalpaper dryer PD is passed through the gas passage of a water-condensingheat exchanger AH, to heat ambient air which passes into and through thetubes of the heat exchanger from an ambient air inlet duct ID, wherebythe ambient air is heated, from an initial temperature of say 30° F. to100° F., up to a temperature of say 380° F. The air leaving the heatexchanger is moved through a duct by blower B to be used as make-up airin the dryer apparatus, and having been significantly pre-heated in heatexchanger unit AH, substantially less fuel is required to conduct thedrying process. As in the case of the water heating systems previouslydiscussed, sulfuric acid condenses at one level within the heatexchanger. The condensation entraps paper particles as well as otherparticulate matter, with incipient tendencies of causing severecorrosion and clogging, but control of flow rates in relation totemperatures in accordance with the invention, so that large amounts ofwater vapor are condensed at a higher elevation within the heatexchanger causes a rain to wash away the sulfuric acid-particulatematter composition. And as in the case of water heating applications,use of the water condensing mode increases the amount of sensible heatrecovered, provides recovery of significant amounts of latent heat, andmaintains heat exchange surfaces wet and clean to improve heat transfer.

In various applications, the hot exhaust gas supplied by a furnace orother device may have an initial temperature substantially exceedingthat (e.g. 550° F.) to which the fluoroplastic protective coatings cansafely be exposed, but that by no means rules out use of the inventionin such applications. In FIG. 3c exhaust gas emanating from anindustrial furnace IF and assumed to have a temperature of 950° F., ispassed to a conventional prior art preheater AP which need not havecorrosion-protective coatings. The air preheater AP also receives airfrom a heat exchanger HXA operated according to the invention in thewater condensing mode. Heat exchanger HXA heats ambient air up to atemperature (e.g. 360° F.) substantially above that at which sulfuricacid will condense, and hence conventional preheater AP does notexperience corrosion or clogging. Preheater AP further raises thetemperature of the 360° F. air up to a higher temperature, e.g. 550° F.,and that air is shown supplied to the burner of furnace IF. In heatingthe combustion air from 360° F. to 550° F., the flue gas passing throughunit AP cools from 950° F. to 425° F., the latter being a temperaturewhich the corrosion-protective coatings in heat exchanger HXA canreadily withstand. With ambient air entering unit HXA at 60° F., verysubstantial condensation of water vapor from the flue gas occurs in unitHXA, and the same advantages of its water condensing mode of operationas previously discussed are obtained. In FIG. 3c it is not necessarythat the heated air (shown at 550° F.) be supplied to the same device(shown as furnace IF) which produces the initial hot exhaust gas (shownat 950° F.); i.e. the heated air could be used for a completelydifferent industial process, but the arrangement shown is believed to bean advantageous and natural use of the invention. While the descriptionof FIG. 3c refers to sulfuric trioxide as a specific condensablecorrosive constituent in the exhaust gas, it will be apparent that theprinciples of FIG. 3c well may find application with exhaust gases whichcontain other potentially corrosive constituents which condense attemperatures in between the material limit operating temperature of thecorrosion-protective coatings and the water vapor condensationtemperature. A conventional non-condensing heat exchanger can be used tolower an exhaust gas temperature below the material limit operatingtemperature of the condensing heat exchanger in numerous applicationswhere, unlike FIG. 3c, the fluid heated by the condensing heat exchangerdoes not pass through the conventional heat exchanger, and that fluidheated by the condensing heat exchanger can be water, of course, ratherthan air. In FIG. 3d exhaust gas from the stack of furnace F2 passesthrough a conventional air preheater AH, and thence through awater-condensing heat exchanger HXB operated according to the invention,to heat water circulated through unit HXB via lines 20,21. A thermalsensor TS2 senses the gas temperature entering unit HXB and controls theflow of fluid being heated by the conventional heat exchanger AH,increasing that flow to decrease the temperature of the gas enteringunit HXB should it begin to rise above a desired value. The thermalsensor is depicted as controlling the speed of a blower motor BM via amotor controller MC, but it will be apparent that the thermal loadimposed on unit AH may be varied in other ways in various applications,such as by positioning of a damper valve which controls flow of thecooler fluid through unit AH. While FIGS. 3c and 3d illustrate uses ofwater-condensing heat exchangers with conventional or prior artnon-condensing heat exchangers, it is to be understood that awater-condensing heat exchanger and a non-condensing heat exchanger canbe combined in the sense of being mounted adjacent each other or on acommon support so as to shorten or eliminate ducting between the twoheat exchangers. The usual flue gas exit temperature from manyconventional economizers and air preheaters is maintained at about 350°F. to avoid condensation of SO₃. It is common at many steam generatingplants to route boiler furnace gas successively through an economizer,an air preheater and a bag house to a stack. In order to avoid seriouscorrosion in the air preheater it has been necessary to preheat ambientair before it enters the conventional air preheater. Such preheatingconventionally has been done by means of steam coils in the inlet airduct. In accordance with the invention, such steam coils may beeliminated. A water-condensing heat exchanger installed to receive fluegas which has passed through the conventional air preheater may be usedto heat ambient air and supply it to the conventional air preheater at asufficiently high temperature that no condensation or corrosion willoccur in the conventional air preheater. Many industrial boilers useeconomizers to heat boiler feed water, and in many cases such boilersrequire 50-75% cold makeup water. The water-condensing heat exchanger ofthe invention may be installed to receive the flue gas exiting from theeconomizer at say 350° F., which normally has been expelled through astack, to heat the boiler makeup water before the water goes to thede-aerator associated with the boiler.

It should be understood that while temperatures of the order of500°-550° F. have been mentioned as suitable upper limits with currentlyavailable fluoroplastic materials, that such limits may rise as thethermal properties of Teflon are improved in the future, or as othermaterials having higher material limit operating temperatures becomeavailable.

Because the heat transfer surface area which is required varies widelybetween different applications, it is highly desirable thatwater-condensing heat exchangers be made in modular form. FIGS. 4a and4b illustrate one exemplary module. Outwardly facing channel-shapedmembers 40,41 of formed sheet steel form rigid side members, and carrybolt-holes 42,42 in upper and lower channel flanges, allowing as manymodules as may be needed to provide a desired heat transfer surface areato be stacked vertically atop one another and bolted together. The unitincludes two tube sheets or end plates 43,44 which are bolted to theside members 40,41, as by means of bolts 45,45. Each tube sheet isprovided with four outwardly-extending flanges along its four respectiveedges, such as flanges 43a to 43d shown for tube sheet 43 in FIG. 4b. Inthe module depicted each tube sheet carries 8 rows of holes, with 18holes in each row, with the holes in alternate rows staggered as shown,and 144 tubes 48,48 extend between the two tube sheets, extending about2.81 inches outside each tube sheet. In one successful embodiment theoutside diameter of each tube, with its corrosion-protection coating was1.165 inches, the tubes in each row were spaced on 1.75 inch centers,the tubes in successive rows horizontally staggered 0.875 inch, and thevertical distance between tube centers was 1.516 inch, as shown in FIG.4c, making the center-to-center distance between tubes in one row andtubes in an adjacent row also 1.75 inch. In FIG. 4c the centers of thethree tubes shown, two of which are in one row and the other of which isin an adjacent lower row, lie on the vertices of an equilateraltriangle. With that tube size and horizontal spacing, it will be seenthat a view vertically through the module is fully occluded by thetubes, except for narrow (e.g. 7/16 in.) strip spaces adjacent thechannel side members 40,41. Except in those narrow strip spaces upwardpassage of gas necessarily requires the gas to be deflectedhorizontally, promoting turbulence, and condensate which drips from atube in any upper row tends to drip onto the center of a tube which istwo rows lower. With the length of each tube inside the module equal to54 inches, and the outside diameter of each tube equal to 1.125 inch,each tube has a surface area inside the module of 1.33 sq.ft., providinga heat transfer surface for all 144 tubes of 190.8 sq.ft. within avolume of approximately 13.5 cu.ft. Each tube comprised a Type L (0.050in. wall thickness) copper water tube of nominal 1 inch inside diameterhaving an actual inside diameter of 1.025 inch and an outside (uncoated)diameter of 1.125 inch. Each tube was covered with a 0.020 inch (20mil.) thick layer of fluoroplastic, namely FEP "Teflon" (fluorinatedethylene propylene). The inside wall of each channel member and theinside walls of the two tube sheets were lined with 0.060 inch (60 mil.)thick fluoroplastic, namely tetrafluorethylene (TFE) "Teflon", and henceall surface area within the module is protected by a thin fluoroplasticcovering. Tube sheet 43 carries a 60 mil layer of tetrafluorethylenefluoroplastic not only on its inside surface visible in FIG. 4b, butalso on the upper surface of its upper flange 42a, the lower surface ofits lower flange 42d, and the leftside and rightside (in FIG. 4b)surfaces of flanges 42b and 42c, and tube sheet 44 is covered withTeflon in the same manner. Channel shaped side members 40,41 carry a 60mil layer of tetrafluorethylene fluoroplastic not only on their vertical(in FIG. 4b) inside surfaces 40a,41a, but also on the top surfaces40b,41b of their upper flanges and the lower surfaces 40c,41c of theirlower flanges. Thus fluoroplastic-to-fluoroplastic joints exist betweenthe channel-shaped side members 40,41 and the side flanges of the tubesheets where they are bolted together. The fluoroplastic coverings forthe tube sheets and side members are formed by cutting fluoroplasticsheet material to size, and then heating it sufficiently to make the90-degree bends necessary to cover the flanges. Three modules of thetype described were vertically stacked, to provide 572 sq.ft. of heattransfer surface area, in a system intended to handle a flue gas flow of9660 lbs.per hour, with a gas mass flow of 0.762 lbs.per second persq.ft. of open gas passage area. The velocity of the flue gas within theheat exchanger should be high enough to provide turbulent flow to insuregood heat transfer, but not so high as to cause abrasion offluoroplastic coatings or to blow large amounts of condensate up thestack. Velocities within the range of 10-40 feet per second have provensuitable in the unit described. It should be recognized that smaller orlarger velocities may be quite suitable in many applications of theinvention.

The corrosion-prevention covering has been applied to the heat exchangertubes by heat-shrinking, or extruding techniques generally well known.After buffing a straight section of copper (or aluminum) tube to cleanit and to remove any burrs, tetrafluorethylene fluoroplastic tubingapproximating the length of the metal is slid over the metal tube. Next,a short end length portion of several inches of the metal tube, coveredwith the fluoroplastic tubing extending slightly beyond the end of themetal tube, is immersed in a tank of propylene glycol heated to 330° F.Only the short length is immersed and heated for several seconds,causing the fluoroplastic tubing to shrink tightly about the shortlength of immersed metal tube. Only after the short end length offluoroplastic has shrunk, is it safe to further immerse the assembly;otherwise heated propylene glycol might enter between the fluoroplastictubing and the exterior surface of the metal tube. Once the short endlength of tubing has shrunk, the metal tube-fluoroplastic tubingassembly may be lowered further into the propylene glycol bath to itsfull length, typically at a rate of about 1 foot per second. Because theheated propylene glycol flows inside the metal tube as well assurrounding the outside of the fluoroplastic tubing, uniform heating andshrinking occurs. After the full length has been immersed for 2-4seconds the assembly may be removed from the tank. The fluoroplastictubing increases in length as it shrinks in diameter, so that after heatshrinking it extends beyond the ends of the metal tubing, and may be cutoff. The corrosion-prevention coverings alternatively can be applied toheat exchanger tubes by using hot air to heat shrink the coverings, or,the coverings may be extruded directly onto the tubes.

After a sheet of fluoroplastic has been bent to surround the flanges ofa tube sheet, holes are punched through the fluoroplastic sheetconcentric with the holes in the tube sheet, but with a smallerdiameter. The holes in each steel tube sheet each have a diameter (e.g.1.28 inch) which exceeds the outside diameter of the covered tubes by0.060 in. (60 mils) the thickness of the fluoroplastic covering on thetubes, but the holes punched in the fluoroplastic tube sheet are eachsubstantially smaller, e.g. 0.625 inch in diameter, as is shown in FIG.7a, where portions of the tetrafluorethylene fluoroplastic sheet 101initially cover portions of a hole in tube sheet 43. The edge of thehole in the tube sheet is preferably made slightly beveled on the insideof the tube sheet, as best seen at 102 in FIG. 7b, but flat orperpendicular on the outside of the tube sheet, as shown at 103. Next, atapered electrically heated tool 104 is used to extrude portions of thefluoroplastic sheet through the holes in the steel tube sheet. The metaltool 104 has a rounded nose small enough (e.g. 0.375 inch) to enter a0.627 inch hole in the fluoroplastic sheet 101, and rearwardly from thenose the tool tapers gradually upwardly to a constant diameter dequalling the outside diameter of the fluoroplastic-covered tubes to beused. The tool is heated to 780° F. With the tool nose inserted into ahole in the fluoroplastic sheet, as shown in FIG. 7a, tool 104 is urgedlightly against the fluoroplastic sheet by an air cylinder (not shown).As the tool heats the edges of the hole in the fluoroplastic, the toolis gradually advanced by the constant force from the air cylinder. Asthe constant diameter portion of the tool nears the fluoroplastic sheet,tool advancement tends to slow or stop until the fluoroplastic isfurther heated, and then the tool suddenly pushes through, after whichfurther tool advancement is prevented by a stop (not shown). Thefluoroplastic then lines the hole in the metal tube sheet with a 30 milthick lining, with some fluoroplastic extending on the outside of thetube sheet. The tool is held extending through the tube sheet for about15 seconds while fluoroplastic on the outside of the tube sheet furtherheats, and then the tool is rapidly retracted out of the tube sheet.Retraction causes a collar or bead having a diameter exceeding that ofthe hole in the tube sheet to be formed around the hole on the outsideof the tube sheet, as indicated at 105 in FIG. 7b. The rounding orbeveling of the tube sheet hole on its inside edge helps prevent thefluoroplastic sheet from cracking as the tool is forced into the hole.The perpendicular edge of the hole on the outside of the sheet tends toimpede inward flow of soft fluoroplastic as the tool is retracted, andconsequent forming of the collar or bead 105.

Immediately (e.g. within 6 seconds) after the heated tool 104 isretracted, a plug having the same outside diameter as thefluoroplastic-covered tube to be used is inserted into the tube sheethole through which fluoroplastic has been extruded, to prevent anydecrease in diameter. Cylindrical plugs formed of many differentmaterials can be used, but short lengths of fluoroplastic-covered copperor aluminum tubing cut from a tube covered as previously described arepreferred. Thus FIG. 7b can be deemed to illustrate a tube sheet holecarrying such a plug, if item 48 is deemed to be a shortlength offluoroplastic-covered tube.

When all of the holes in two tube sheets have been processed in such amanner, a pair of side members 40,41 and a pair of tube sheets arebolted together into their final configuration. Then thefluoroplastic-covered tubes are slid through mating pairs of holes inthe two tube sheets in the following manner. The plug is removed from ahole in tube sheet 43, one end of a tube 48 is promptly inserted intothe hole from the outside of the tube sheet 43, and the tube promptlyurged inwardly until its entry end nears tube sheet 44 at the other endof the module. The tube can be manually urged through the hole in tubesheet 43 if that is done within 2 or 3 minutes after the plug has beenremoved. As the entry end of the tube nears tube sheet 44, the plug inthe proper hole in tube sheet 44 is removed by another person at thetube sheet 44 end of the module, and the entry end of the tube can bepushed through the hole in tube sheet 44 to its final position, with twopersons at opposite ends of the module pushing and pulling on the tube.That it generally requires two strong persons to slide the tube when itis installed in both tube sheets indicates the tightness of the fitwhich occurs. Hydraulic rams of the like can be used, of course, tofacilitate insertion of the tubes. While insertion of a tube can takeplace over a time period of several (e.g. 3) minutes, use of less timetends to be advantageous. But in any event, insertion of the plugs intothe fluoroplastic-lined holes in the tube sheets to prevent diametricalreduction until no more than a few minutes before a tube is installed,is deemed very important. With a plug maintained in eachfluoroplastic-lined tube sheet hole from the time when the Teflon isextruded through the hole until just before a tube is inserted in thehole, no diametrical reduction of the fluoroplastic-lined hole beginsuntil the plug is removed. Diametrical reduction occurs slowly enoughafter a plug has been removed that there is time to urge a tube intoplace if it is done promptly enough, and then, importantly, after a tubeis in place further diametrical reduction occurs, so that the(tetrafluoroplastic) collar and hole lining in a given tube sheet holetightly grips the (fluorinated ethylene propylene) layer on the tube,clamping the tube very tightly in place, so that it cannot be removedexcept with extreme force. No other mechanism or clamping devices areused to hold the tubes in place, so that the tubes can be deemed tomechanically float relative to the tube sheets, being clamped only bythe fluoroplastic collars. Such an arrangement has proven to accommodatethe expansions and contractions which heating and cooling cause tooccur, with no loss of integrity in the fluoroplastic-to-fluoroplasticseals.

In order to provide one water inlet and one water outlet for a heatexchanger constructed of modules of the nature shown in FIGS. 4a-4c, itis necessary, of course, to provide return bend connections on the endsof some of the tube ends extending out of the tube sheets. ConventionalU-shaped copper return bends are sweated on the ends of the tubes tomake such connections. The equilateral triangle spacing of the tubesadvantageously allows a single type of return bend connection to be usedeither to connect tubes in the same row or to connect tubes in adjacentrows. While it might be possible in some applications to provide waterflow serially in one path through all of the water tubes, mostapplications will utilize manifolding into plural water paths, toprovide better and more uniform heat transfer, and to avoid tube erosionfrom high water velocities. For example, in a system using three modulesof the 18 by 8 tube matrix shown in FIGS. 4a-4c, water was introducedinto nine of the 18 tubes in the uppermost row and directed through theremaining nine tubes in that row through a series of return bends,providing flow paths as diagrammatically indicated in FIG. 4e, wherearrows indicate inlet flow from a simple manifold MA (e.g. a 3-inchpipe), and circles represent connections to tubes of the adjacent lowerrow. Nine separate flow paths through the heat exchanger were providedin such a manner to accommodate water flow of 70 gallons per minute at awater velocity kept under 4 feet per second to avoid erosion. Theoverall water flow rate which is desired may vary widely in differentapplications. Because the tubes are straight inside the module, with allreturn bends connected outside the tube sheets, modules fabricated to beidentical ultimately may be used to accommodate flow rates within alarge range, which advantageously leads to economies in fabrication andstocking.

When most heat exchangers utilize return bends inside a heat exchangerchamber or housing, the use inside the module of FIGS. 4a-4c of onlystraight cylindrical sections of tubes has further significantadvantages. Cleaning of the tubes by the falling condensate is morethorough and uniform because no return bends or extended surfaces areused. The use of solely straight sections also makes heat-shrinking orextrusion of fluoroplastic of the tubes practical.

In FIGS. 5a and 5b exhaust gas is conducted via inlet duct 50 into thelower plenum 51 of a water-condensing heat exchanger for upward passagebetween nests of fluorinated ethylene propylene fluoroplastic-coveredaluminum tubes into fiberglass upper plenum 56 and out fiberglass stack57. Each tube extends through the two tube sheets supporting its ends.An inlet air duct 52 covers one end of an uppermost group of tubes 62.The other ends of tube group 62 and the ends of a next lower group oftubes 63 are shown covered by hood or cover 63, so that air exiting fromtube group 62, rightwardly in FIG. 5a, is returned through tube group64, leftwardly in FIG. 5a. Similar hood means 65-69 similarly reversethe direction of air flow at opposite sides of the assembly. Air fromthe lowermost group of tubes 70 passes into outlet duct 71. FIGS. 5a and5b illustrate a system in which air passes through the heat exchangerseven times, for sake of illustration. In actual practice one to fivepasses ordinarily has been deemed adequate. The insides of the tubesheets, slide members, and the covers, and the inside of plenum 51, arelined with corrosion-protection lining, as in the case of water-heatingexchangers. Though not shown in FIG. 5a and 5b, it will be apparent atthis point, that if desired, spray nozzles can be provided inside theair-heating heat exchanger of FIGS. 5a and 5b for the same purposes aswere mentioned in connection with the water-heater heat exchanger ofFIG. 3a.

In the system shown in FIG. 6 a water condensing heat exchanger BHXarranged to heat both combustion air and boiler makeup water comprisessix vertically-stacked modules 61-66. Ambient or room air enters module63 and the upper half of module 62 through an inlet duct 67, makes onepass across the heat exchanger, and is directed by return plenum or hood68 back through the tubes in module 61 and the top half of module 62,into ducting 70 which connects to the inlet of a forced draft fan FDF.

The upper three modules 64-66 of heat exchanger BHX preheat boilermakeup water. The water flows from a cold water main source 71 to aninlet manifold IM which distributes the water laterally across the toprow of tubes in module 66 in seven water flow paths which progresshorizontally and vertically through modules 64-66 to outlet manifold OM.Flue gas enters lower plenum 72 of heat exchanger BHX through duct 73,and passes upwardly through modules 61 to 66 in succession, intofiberglass upper plenum 74 and thence out fiberglass stack 75. The heatrecover system of FIG. 6 is intended to accommodate flue gas, combustionair and makeup water flow rates prevailing in a boiler system having twoboilers B1,B2, which system produces a maximum of 50,000 pounds per hourof steam firing No. 6 fuel oil at 15% excess air and with 67% makeupwater, and an average of 30,000 pounds per hour. Flue gas is pulled fromthe stacks of the two boilers by a single induced-draft fan IDF, withthe amount of flue gas being drawn from each boiler being controlled bya respective damper, 76 or 77, which is controlled by a respectivemodulating positioner, MP1 or MP2, and the modulating positioners areeach controlled by the load on a respective boiler using conventionalpneumatic control signals from a conventional boiler control system. Themodulating positioners are set so that under full load conditionsdampers 76 and 77 are fully open and all of the flue gas produced byeach boiler is directed to heat exchanger BHX. If fan IDF were to pullmore flue gas than both boilers are producing, either the boiler excessair would increase or outside air would be drawn down the boiler stacks,in either case decreasing efficiency. To avoid those problems, dampers76 and 77 decrease the amount of flue gas drawn from each boiler whenits load is decreased. The modulating positioners MP1 and MP2 closetheir respective dampers when their associated boilers are shut down,and they are interlocked with induced-draft fan IDF to close theirdampers if fan IDF is not running, and then all flue gas will exitthrough the boiler stacks.

The ducts 78,80 containing dampers 76,77 merge to a common duct 81.Damper 82 in duct 81 regulates the total amount of flue gas going toheat exchanger BHX depending upon the combustion air and makeup waterdemand rates. Damper 82 is modulated to control the temperature of thepre-heated makeup water exiting from exchanger BHX at a desired setpoint (180° F.), by sensing the water temperature at its exit from unitBHX to operate a proportional servomotor SM1. Damper 82 is normallyfully open, allowing all of the flue gas being produced by the boilersto pass through heat exchanger BHX, but during rapid transientconditions, or during periods of reduced makeup water requirements, moreheat may be available from the flue gas than can be utilized to preheatthe combustion air and makeup water, in which case the temperature ofwater exiting from heat exchanger BHX will start to rise, but servomotorSM1 then will begin to close damper 82 to maintain the exit temperatureof water from heat exchanger BHX at the desired setpoint. Since the fluegas passes first through the lower air-heating section and then throughthe water-heating section, where the input water temperature (e.g. 46°F.) is lower than the input air temperature (e.g. 85° F.), maximum heatrecovery is obtained. A further modulating damper 83 ahead of theinduced draft fan operates to limit the temperature of flue gas byadmitting sufficient room air to mix with the flue gas that the inlettemperature of the flue gas to heat exchanger BHX does not exceed thesafe operating temperature (e.g. 500° F.) for the fluoroplasticcorrosion-prevention materials which line heat exchanger BHX.Temperature sensor TS3 senses the flue gas temperature at the exit offan IDF to control the position of damper 83 via servomotor SM2. If thetemperature of the flue gas is 500° F. or less, damper 83 will be fullyclosed.

The flue gas passes from fan IDF through a short section of ducting 73into bottom plenum 72, and thence upwardly through heat exchanger BHX,first heating combustion air and then heating boiler makeup water, andcopious condensation occurs, providing numerous advantages effectsheretofore described. Drain line D at the bottom of unit BHX includes atransparent section of tubing 85 through which the color of thecondensate may be observed.

A dial thermometer DT1 located in stack 75 indicates flue gas exittemperature, which typically varies between 90° F. and 200° F.,depending upon boiler load. Temperature sensor TS4 also located at stack75 serves to shut down the heat recovery system by closing damper 82 ifthe flue gas exit temperature should exceed 200° F.

Simple BTU computers receive water and air temperature and flow ratesignals and compute the amounts of heat recovered. At an average steamload of 30,000 pounds per hour, the combined amount of heat recovered is3,463,000 Btu per hour. Prior to use of the condensing heat exchangerthe average steam load of 30,000 pounds per hour required an averagefuel consumption of 256 gallons per hour of No. 6 fuel (148,000 Btu pergallon) with a boiler efficiency of 80%. Utilizing the water-condensingheat exchanger 29.3 gallons per hour of fuel are saved, a savings of11.5%.

The lower housing (11 in FIG. 1, for example) comprises a simple steelsheet housing completely lined with 60 mil TFE fluoroplastic, with itsflanges also covered with fluoroplastic in generally the same manner asin the tube modules. The lower housing may take a variety of differentshapes in various applications. An upper section (e.g. 1 foot) of drainD which connects to the bottom of the lower housing is also preferablyformed of TFE fluoroplastic tubing. When No. 6 oil is used as boilerfuel, the condensate is black due to the large amount of particulatematter and SO₃ removed from the flue gas, while use of natural gas asboiler fuel provides a virtually clear condensate due to the very smallamounts of particulate matter in the flue gas.

It is believed that flue gas flow velocities up to 60 feet per secondwill be quite workable, and that flue gas pressure drops across awater-condensing heat exchanger of 3 inches of water should be workable.

It may be noted that the heat exchanger units depicted in FIGS. 1-6 eachhave been described as employing upward flow of the exhaust gas anddownward flow of the fluid (water or air) which is to be heated. I havediscovered that substantially more heat can be extracted from theexhaust (flue) gas in a heat exchanger of given heat exchange surfacearea if both of those flows are reversed, i.e. if flue gas is arrangedto pass downwardly through the chamber and the fluid to be heated passesgenerally upwardly through successive ones of the generally horizontaltubes.

In the heat exchanger of FIGS. 8a and 8b fue gas forced through inletduct 28 by blower BL enters an upper housing 29 and then passesdownwardly through a plurality of tube modules 32-35 into a bottomplenum 36, and then through an outlet duct 37 to a stack (not shown).Each module is shown for sake of simplicity as having only four rows oftubes, and it will be apparent that many more rows can be provided ineach module. Cold water introduced into lower tubes as at 25 flowsgenerally upwardly thrugh successive tubes of successive modules, andhot water exits near the top of the heat exchanger, as at 26. It may beseen in terms of construction that the system of FIGS. 8a and 8b differsfrom that of FIG. 1 almost solely in that the directions of flows of thetwo fluids are reversed in a vertical, or top-bottom sense. Oneinitially would assume that such reversals would have little or noeffect on heat recovery, and that perhaps the system of FIG. 1 mightprovide slightly better heat transfer by reason of the condensate givingup some of its heat as it cascades down over lower tubes. But it hasbeen found that the arrangement of FIGS. 8a and 8b provides a surprisingimprovement in heat recovery over that of the system of FIGS. 1 and 2.To facilitate further discussion, the system of FIGS. 1 and 2 will becharacterized as a "gas upflow" system, and the systems of FIGS. 8a,8b,9and 10 will be called a "gas downflow" system. In the operation of a gasdownflow condensing heat exchanger according to the invention, water (orair) at or below, and preferably well below the flue gas watercondensation temperature e.g. 120° F. (49° C.), is maintained in a groupof tubes near the bottom of the heat exchanger unit, insuring that largeamounts of water vapor condense in that lower portion of the heatexchanger.

It is believed that the gas downflow system provides an improvement inheat transfer of approximately 50% over that of the gas upflow system,so that in a given installation a gas downflow system requires onlyabout two-thirds as much tube surface heat transfer area as a gas upflowunit requires. Such a decrease in required tube surface area drasticallydecreases the cost of a given installation, of course, which hasimportant economic implications. While the principal reason for thesurprising increase in heat transfer is not certain, one or more of thefollowing theories may account for it.

In the gas upflow unit of FIG. 1 where maximum condensation occursadjacent the top of the unit, the fall of condensate through the entirevertical length of the unit causes substantially all of the tubeexternal surface area to continuously remain wet. It is believed to bepossible that such wetting of tubes by the falling condensate in the gasupflow unit of FIG. 1 may act to undesirably shield tubes from the hotexhaust gas, thereby decreasing heat transfer in comparison to that in agas downflow system. Conversely, in the gas downflow unit of FIGS. 8aand 8b, condensation of water occurs only adjacent a bottom group oftubes, say in 10 bottom rows of a unit having 40 rows, for example, sothat any such "shielding" is much less.

In the gas upflow unit of FIG. 1 where maximum condensation of wateroccurs adjacent an uppermost group of tubes, drops of water fall throughthe entire zone within the unit where the average flue gas temperatureis above water condensation temperature, and as they fall they removesome heat from the flue gas stream. As some drops of water reach veryhot flue gas in the portion of the unit, they will be partially or fullyvaporized, and the resulting stream then will travel upwardly with theflue gas until it reaches a level at which it will again condense. Thussome water particles likely repeatedly fall and rise within the gasupflow heat exchanger. Vaporization of a drop of water in the lowerportion of the heat exchanger removes some heat from the flue gas, sothat the average temperature of the flue gas as it passes upwardlythrough the unit is less than it would be if the drops of water were notpresent, and a lesser average flue gas temperature limits the amount ofheat which can be extracted. In the gas downflow system of FIG. 8awherein condensation of water occurs only adjacent a group of tubes atthe lower portion of the unit, virtually all drops of water fall intothe bottom plenum without passing through the zones where flue gastemperature is above water condensation temperature, so that heat is notextracted from the flue gas stream to heat or vaporize drops of water,and hence the average temperature of the flue gas during its travelthrough the heat exchanger is higher than in the system of FIG. 1, andmore heat is extracted from the flue gas.

Assume that the tubes shown at a in FIG. 9 represent various of thetubes in a group of bottom rows of tubes of the gas downflow unit ofFIG. 8a. Further assume that condensation of water occurs generallyadjacent the rows of tubes at a, but that condensation of water does notoccur adjacent tubes above those of group a, such as those partiallyshown at b. Condensation of water will not occur adjacent the tubes ofgroup b if the flue gas temperature is roughly 120° F. or higher in thatarea. Condensation of sulfur trioxide to form sulfuric acid will occuradjacent some of the tubes of group b, however, since it typicallyoccurs at temperatures of the order of 250° F. The amount of sulfuricacid which condenses is very small, of course, in comparison to theamount of water condensed in the lower portion of the heat exchanger.The tubes on which sulfuric acid forms technically might be slightlymoist, but essentially dry compared to those tubes where watercondensation occurs.

But returning now to the bottom group of tubes at a, note that withwater traveling upwardly through the tubes, the water in the tubes inrow #10 is hotter than that in the tubes of row #9, that in the tubes inrow #9 is hotter than that in the tubes of row #8, etc. Thus a drop ofcondensate falling from a tube in one row of group a to the second lowerrow in group a falls to a tube having a lower temperature, and in doingwill transfer some heat to that lower tube. That water-to-water heattransfer, which is potentially much more efficient than gas-to-waterheat transfer, may explain the surprising improvement in heat transferwhich occurs in a gas downflow unit.

While it was initially believed that maximum condensation was requirednear the top of a heat exchanger, to provide a substantial "rain"throughout most of the height of the heat exchanger, if effectiveremoval of particulates from the flue gas was to be accomplished, it isbelieved that effective particulate removal also can be accomplishedwith a gas downflow system. While conclusive test data has not beengathered to evaluate particulate removal in the gas downflow system, itis believed that the gas downflow system may provide even betterparticulate removal than a gas upflow system, for the following reasons.

A dry solid particle will tend to be swept along in, and to remain in,the flue gas stream, no matter the direction of the flue gas stream, ifthe weight of the dry solid particle remains sufficiently small comparedto its effective surface area. If a dry solid particle joins a dropletof moisture, the then wetted particle-droplet combination tends to havea greater weight (or mass) per unit of effective area. If that mass perunit area becomes great enough, the particle-droplet combination willtend to leave the flue gas stream, by reasons of several mechanisms. Forexample, if the flue gas stream were traveling horizontally in a duct ofsufficient length, the particle-droplet combination would eventuallyreach the bottom of the duct by reason of gravitational acceleration. Ifthe gas stream receives a substantial change in direction, say fromstraight down to horizontal, the momentum of the particle-dropletcombination will tend to cause it to continue vertically, into thebottom plenum of a gas downflow unit. In such a case, gravitationalacceleration adds to the momentum provided by gas velocity, aidingseparation of the particle-droplet combination from the flue gas stream.And once a dry solid particle joins a droplet of water in the bottomsection of gas downflow heat exchanger, there is no tendency for thatsolid particle to separate from the water droplet, because theparticle-droplet combination does not thereafter pass through an area inwhich the droplet can be evaporated. If the gas stream insteadprogresses upwardly, gravitational acceleration opposes the momentumwhich the upward gas velocity initially provides. With lessparticle-droplet momentum and with no 90° bend in the gas streamdirection, there is much less chance for momentum to remove aparticle-droplet combination from the flue gas stream. Further, in thecase of a gas upflow unit, a given particle-droplet combination formedadjacent the upper section of the heat exchanger necessarily must fallto and through the lower section of the heat exchanger to be removedfrom the flue gas stream. Since that lower section contains flue gassubstantially above water condensation temperature, the water dropletportions of many particle-droplet combinations will re-evaporate,leaving dry small-mass solids which will be swept up the stack unlessthey later happen to join with other droplets of water which fallthrough the lower section of the heat exchanger without beingre-evaporated.

As previously mentioned, in the gas downflow heat exchanger, sulfuricacid can condense on tubes above those where water condensation occurs.However, the gas velocity continuously tends to blow acid dropletsdownwardly, eventually onto lower tubes where copious water condensationis occurring, or into the bottom plenum 36. With the gas velocity keptreasonably low, at 50 feet per second or less in typical installations,condensed droplets of sulfuric acid tend to fall to the bottom plenum36, until they are washed away by condensed water, rather than beingswept horizontally along outlet duct 37 and up the stack. It may benoted that once a droplet of sulfuric acid condenses in the gas downflowunit of FIG. 8a, it moves only downwardly, toward a lower temperaturearea, and hence it cannot re-separate into SO₃ or H₂ O.

In FIG. 8a the heat exchanger is shown including an optional nozzle 38extending into duct 30 to introduce a scrubbing liquor into the flue gasstream. The liquor reacts with the sulfur dioxide in the flue gas,thereby removing sulfur dioxide from the gas which exits from outletduct 37. With a built-in scrubber, the user can burn a much lower gradeof fuel oil or coal containing substantial sulfur and still meet givenrestrictions on stack emissions. The liquor containing sulfur oxidesfalls to the bottom of the heat exchanger, and the continuouscondensation of substantial amounts of water on the lower group of tubesprevents it from accumulating in the bottom of the heat exchanger. Theintroduction of scrubbing liquor into a flue gas stream to remove sulfurdioxide is not per se novel; however, prior systems utilizing such atechnique required that very large amounts of water be supplied andinjected into the flue gas stream. Use of a water-condensing heatexchanger constructed according to the present invention obviates anyneed for a substantial water source for such scrubbing, as well asproviding the important heat recovery heretofore described herein.

The tube modules used in the gas downflow system of FIG. 8a may beidentical to those described for use with gas upflow condensing heatexchangers. The lower plenum 36 and outlet duct 37 are preferably formedof fiberglass. The upper chamber 31 may be, but need not be formed offiberglass, since no condensation occurs in that chamber. Like gasupflow units, gas downflow condensing heat exchanger units may beconstructed to heat air instead of water, or both air and water, and gasdownflow units may be connected in series with non-condensing heatexchangers in the same manner as has been described above for gas upflowcondensing heat exchangers. The techniques heretofore described forlimiting the temperature to which the fluoroplastic is exposed may beused with gas downflow condensing heat exchangers as well as gas upflowcondensing heat exchangers.

FIG. 10 illustrates the variations of flue gas and water temperature inone successful embodiment of a gas downflow heat exchanger in typicalpractice of the invention to heat water. Temperatures of flue gas andwater are plotted against vertical elevation within a downflow unit withfour heat exchanger modules. Thus the flue gas temperature falls from aninput temperature FG₁ of 260° F. at the top of the heat exchanger to anoutput temperature FG₀ of 93° F. at the bottom of the heat exchanger asthe gas travels downwardly through the heat exchanger, as indicated bycurve FG. The gas temperature plot in FIG. 10 should be understood to beapproximate for all points, and in general to represent for anyelevation the lowest temperature to which substantial portions of thegas are lowered at that elevation. At any elevation below the topmostrow of tubes there are temperature gradients, of course, and the averagetemperature, if averaged over the entire cross-sectional area of theheat exchanger, will be above that plotted as curve FG. Simultaneously,water having an input temperature W₁ of 65° F. at the bottom of the heatexchanger flows upward and is heated to an output temperature W₀ of 95°F. at the top of the heat exchanger as shown by curve W in FIG. 10. Thevertical scale in FIG. 10 represents modules Z1 to Z4 of the heatexchanger. A vertical dashed line S at 250° F. represents a typicaltemperature FG₂ at which sulfur trioxide condenses out of the flue gasas sulfuric acid from flue gas formed by the burning of No. 6 fuel oil.

Following the flue gas temperature curve FG for downward flue gas flow,it can be seen that the flue gas cools to the sulfuric acid dewpoint FG₂(250° F.) in zone 4 of the heat exchanger. As the gas exits zone 4, ithas a temperature of about 180° F. Thus most if not all of the sulfuricacid will have condensed by the time the flue gas enters zone 3. As thegas passes through zone 3, the heat transfer is essentially sensible asthe temperature is above the dewpoint of water. The solid vertical lineat 103° F. represents the dewpoint of water at the time of the tests. Asthe gas enters zone 2, the heat transfer is still sensible, but as theflue gas cools to 103° F. water starts to condense and the heat transferbecomes latent. Passing from zone 2 to zone 1, the flue gas cools to atemperature FG₀ of 93° F. as it exits the heat exchanger unit.

The sulfuric acid condensate formed in zone 4 falls to the bottom of theheat exchanger under the influence of gravity and aided by the force ofthe flue gas' downward flow. As the sulfuric acid falls through zones3-1, the temperature inside the heat exchanger is always below thedewpoint temperature for sulfuric acid. This prevents the acid fromre-evaporating and escaping through the exhaust stack as a gas. Watercondensing in zones 2 and 1 also falls to the bottom of the heatexchanger where it dilutes the sulfuric acid and carries it out of theunit through the condensate drain.

In various applications it becomes necessary or desirable to provideheat exchanger modules generally of the type heretofore described, butwhich have substantially greater length dimensions, such as 20 feet. Asthe lengths of modules exceed a given dimension, it becomes necessary ordesirable to provide one or more supports for the tubes in between thetube sheets to prevent sagging and fatigueing of the copper tubes.

The module shown in plan view in FIG. 11a comprises first and secondtube sheets 43',44' which may take the same form as tube sheets 43 and44 of FIGS. 4a and 4b and have their insides and four respective flangescovered with a fluoroplastic as previously described. Tube sheets43',44' are interconnected by successive pairs of side members 140,141and 142,143. Each of the side members comprises a generallychannel-shaped piece of sheet metal having an integral tab portionfilling its recess at one end, as shown at 140a and 142a, for example.The inner sides of the side members are covered with a fluoroplasticsheet which is formed to cover the tops of the upper flanges of the sidemembers and the bottoms of the flower flanges of the side members. Thetab portion 140a of member 140 is bolted to the tab portion of sidemember 142 by a plurality of bolts, such as those shown at 150,150, andside member 141 is similarly bolted to side member 143, with anintermediate support plate 144 sandwiched in between the pairs of sidemembers.

As is shown in FIGS. 11b and 11c, support plate 144, which is preferablyformed of aluminum, is sandwiched between two sides 145a,145b of foldedfluoroplastic sheet 145, the edges of which extend beyond the lower edgeof sheet 144, and which are heat-welded together below those edges. Suchheat welding of sheet 145 prevents flue gas from reaching the upper andlower edges of support sheet 144. Sheet 145 extends laterallysubstantially co-extensively with sheet 144. When support plate 144 andsheet 145 are bolted between the side member pairs as shown in FIG. 11d,the vertically-extending side edges of sheet 144 are not subjected toflue gas; hence sheet 145 need not extend beyond or be formed around theside edges of sheet 144, though it may be so formed, if desired.

Support plate 144 includes a plurality of holes 48', 48' through whichthe fluoroplastic-covered tubes of the module eventually extend. Afterthe folds of sheet 145 have been heat-welded together to locate plate144 therebetween, undersize holes 48 are punched through the folds ofsheet 145, concentric with the holes in support plate 144. A heated dieof the nature of die 104 in FIG. 7a is then forced through each of theconcentric trios of holes. The die is forced in opposite directionsthrough various of the trios of holes, preferably in opposite directionsthrough any adjacent trios of holes. In FIG. 11c wherein the die wasforced upwardly as viewed in that figure, a portion of sheet 145a isurged through the hole 48a' to form a seal at 147 generally similar tothat shown in FIG. 7b, and a portion of sheet 145b is formed to extendaway from the support plate 144, as at 148. In adjacent hole 48b',through which the die was forced downwardly as viewed in the figure, aportion of sheet 145b is forced through hole 48b', and a portion ofsheet 145a is formed to extend away from the support plate 144. Plugmeans (not shown) are placed in the holes as soon as such forming hasbeen completed, to prevent holes 48a, 48b in sheets 145a, 145b fromshrinking in diameter, and the plugs are removed just beforefluoroplastic-covered tubes TU are urged through holes 48. The tubes TUthen will be tightly gripped by both sheets 145a and 145b, as well asbeing gripped at tube sheets 43, 44 (FIG. 4b).

While the module in FIG. 11a is shown as comprising only two pairs ofside members 140, 142 with a single intermediate support plate 144sandwiched between them, it will be apparent that added pairs of similarside members and added support plates of the same type may be cascadedto form heat exchanger modules of any desired length, and to allow useof tubes of very great lengths.

In FIG. 11b, the edges of sides 145a, 145b of sheet 145 are heat-weldedtogether with a 20 mil thick strip 146 of PFA fluoroplastic filmsandwiched in between them. Sheet 145 comprises TFE fluoroplastic. Strip146 is used because pieces of TFE fluoroplastic will not weld together.Support plate 144 could comprise a steel sheet, if desired.

In FIG. 8c, which illustrates a horizontal gas flow condensing heatexchanger, similar reference numerals are used for parts similar tothose in FIGS. 8a and 8b. Hot flue gas introduced via duct 28 intoplenum 29 by means of a blower (not shown) passes generallyhorizontally, rightwardly, but very slightly downwardly, successivelythrough four modules 32-35 into outlet plenum 36 and then out throughstack 37. Cold water introduced at pipe 25 passes successively throughhorizontally extending tubes in the modules, and hot water exits at pipe26. The U-shaped couplings which interconnect the ends of pairs of thehorizontally-extending tubes are shown at 63a, 63a. The flow of flue gaswill be seen to be perpendicular to the horizontal direction in whichthe tubes extend. The water tubes are preferably interconnected so thatwater flow is divided into a plurality (e.g. 4) of flow paths stackedone above another. Condensation of water from the flue gas occursprincipally in the rightward or exit module 35. Condensate dripping fromthe upper tubes of module 35 may effect some shielding of lower tubes inthat module so as to slightly decrease the heat transfer, but in anamount much less than in a gas upflow unit. An optional nozzle apparatusfor introducing a scrubbing liquor is shown at 38. The modules 32-35 maybe constructed identically to those described in connection with FIGS.4a and 4b, or, they may comprise modules of substantially lesser width(lesser height in FIG. 8c). For example, while the modules shown in FIG.8c each contain twelve rows, it will be apparent that fewer rows butmore modules could be provided, resulting in a heat exchanger havingless height but a greater horizontal dimension, which is oftenadvantageous when one must fit a condensing heat exchanger into anexisting boiler installation. Condensation of water from the flue gasthen may be arranged to occur in the last several of the lower-heightmodules. With such lower height modules, falling condensate shieldsfewer tubes. Further, condensate falling from one tube always falls to atube carrying water which is cooler, or about the same temperature, sothat the system of FIG. 8c may operate in a manner quite similar to theapparatus of FIGS. 8a and 8b. The gas entry end of the heat exchanger issupported slightly higher than the gas exit end of the heat exchanger,giving the unit a tilt of the order of 10 degrees, so that condensatefalling downwardly will run rightwardly (in FIG. 8c) to drain D. Thesharp right-angle turn which the gas flow experiences at bottom plenum36 and stack 37 tends to aid separation to particulates from the gasstream. Inasmuch as the gas, water flow and condensate fallingconditions in the device of FIG. 8c should rather closely resemble thoseof the gas downflow heat exchanger of FIGS. 8a and 8b, it is expectedthat the heat transfer of the FIG. 8c device will be very nearly thesame.

An improved flue gas flow control system illustrated in FIG. 12aincludes a differential temperature controller DTC responsive to twotemperatures T₁ and T₂ sensed by temperature probes TA and TB.Temperature probe TA is located in stack S in between the boiler and theconnection of duct 81 to the stack, so that probe TA measures boilerflue gas output temperature (or that from an economizer, not shown, ifone is used). Probe TB is located in duct 81, and it measures a lowertemperature. The normal function of controller DTC is to control damper76 so that all of the flue gas produced by boiler B1, plus a smallamount of outside air drawn down stack S, are drawn by induced draft fanIDF and supplied to condensing heat exchanger BHX. The heat exchangershown in FIG. 12c is assumed, for sake of explanation, to heat bothwater and air. At a reference steady-state operating condition, suchflow of flue gas and air provides a given difference ΔT betweentemperatures T₁ and T₂, the latter temmperature, of course, being lower.

As boiler load increases to provide more flue gas from the boiler,temperature T₂ tends to increase, by reason of flue gas constituting agreater percentage of the mixture being drawn into duct 81. Flue gastends to constitute a greater percentage of the mixture because there isless restriction to flow of flue gas into duct 81 than restriction toflow of outside air down the length of stack S. As temperature T₂increases, making the temperature difference ΔT smaller, a signal fromcontroller DTC commensurate with temperature difference ΔT operates toopen damper 76. When damper 76 opens sufficiently, enough cool air fromthe stack mixes with all of the flue gas produced by the boiler torestore the temperature difference. Conversely, as boiler load decreasesto provide less flue gas, temperature T₂ tends to decrease by reason offlue gas constituting a lesser percentage of the mixture being drawninto duct 81, and the increased temperature difference ΔT causescontroller DTC to act to close damper 76. Thus the system of FIG. 12tends to draw all the available flue gas from the boiler, andimportantly, to do so without affecting normal operation of the boiler.

In FIG. 12a the signal from differential temperature controller DTC isshown applied to control positioner MP1 through a low signal selectorLSS and two pairs of contacts C1, C2. Selector LSS shown also receivinga signal from air temperature controller ATC and a signal from watertemperature controller WTC. Controller ATC provides a signalcommensurate with the temperature of heated air leaving heat exchangerBHX, and controller WTC provides a signal commensurate with thetemperature of heated water leaving heat exchanger BHX. Selector LSS isoperative to select the smallest of the three signals applied to it andto use that selected signal to control the position of damper 76. Inordinary operation, the signal from controller DTC controls the positionof damper 76, but if outlet water temperature or outlet air temperatureexceeds a given respective setpoint, selector LSS uses the output fromcontroller WTC or controller ATC to partially close damper 76, therebylimiting the water or air outlet temperature. A pair of contacts C1 areopened if the boiler is shut off for any reason, and a pair of contactsC2 are opened if fan IDF is not running, and the opening of either pairof contacts causes damper positioner MP1 to fully close damper 76.

FIG. 12b illustrates a system generally similar to that of FIG. 12aexcept that the heat exchanger CHX of FIG. 12b is fed flue gas from twoseparate boilers B10 and B11. Each boiler is provided with respectivegas flow control system (GFC1 or GFC2) similar to gas flow controlsystem GFC of FIG. 12a, a respective damper and modulating positioner,and a respective low signal selector. The water temperature controllerWTC in FIG. 12b applies its signal to both low signal selector circuits,and the air temperature controller ATC applies its signal to both lowsignal selector circuits.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained. Sincecertain changes may be made in carrying out the above method and in theconstructions set forth without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. The method of recoveringheat energy contained in a hot exhaust gas containing water vapor andsulfur trioxide combinable in part to form sulfuric acid, and ofsimultaneously removing substantial amounts of said sulfur trioxide fromthe exhaust gas, which comprises the steps of: passing said exhaust gasin heat exchange flow relationship generally perpendicularly to andbetween generally horizontally extending tubes of a tube nest having aplurality of tube groups; and simultaneously passing a fluid to beheated through successive tubes of said nest in counterflow relationshipto said exhaust gas flow, the flow rate of said exhaust gas beingestablished in relation to the input temperature and flow rate of saidfluid and the heat exchange surface area of said nest so that thetemperature of said fluid within a first group of said tubes remainssubstantially below the water vapor dewpoint of said exhaust gas,thereby transferring a substantial portion of the latent heat energy insaid hot exhaust gas to said fluid and causing continuous substantialcondensation of water from said exhaust gas and a continuous fall ofwater droplets from tubes of said first group through said exhaust gasto the tubes of a lower tube group, whereby said water droplets entrapand dilute sulfuric acid as they fall from said tubes of said firstgroup through said exhaust gas, and said water droplets wash sulfuricacid from said tubes of said lower group, the exterior surfaces of saidtubes of said tube nest being covered with coverings which both shieldsaid tubes from sulfuric acid and promote dropwise condensation.
 2. Themethod of claim 1 wherein said coverings each comprise a layer offluoroplastic having a thickness within the range of 2-30 mils.
 3. Themethod of claim 1 wherein the amount of latent heat energy released bycondensation exceeds 5 percent of the total heat energy transferred tosaid fluid.
 4. The method of claim 1 wherein said step of passing saidexhaust gas comprises forcing said exhaust gas through said tube nest byuse of a blower means.
 5. The method of claim 1 wherein said exhaust gasalso initially contains particulate matter, whereby said water dropletsentrap a portion of said particulate matter as they fall from said tubesof said first group through said exhaust gas.
 6. The method of claim 1which includes the step of introducing a scrubbing liquor into saidexhaust gas prior to passage of said exhaust gas through said tube nest.7. The method of claim 1 which includes the step of varying the amountof said exhaust gas being passed between said tubes of said tube nest tomaintain the temperature of gas exiting from said tube nest at apredetermined value.
 8. The method of claim 1 wherein said exhaust gasis passed generally downwardly through said tube nest, and said fluidbeing heated is passed through successively upward tubes of said nest,whereby said tubes of said first group contain warmer fluid than thetubes of said lower group, and the fall of said water droplets transfersheat to said tubes of said lower group.
 9. The method of claim 1 whereinsaid exhaust gas is passed generally upwardly through said tube nest,and said fluid being heated is passed through successively downwardtubes of said nest.
 10. The method of claim 1 wherein said exhaust gasis forced generally horizontally through said tube nest.
 11. The methodof claim 1 which includes the step of substantially altering thedirection of flow of said exhaust gas upon exit of said gas from saidtube nest, thereby to separate condensation from the exhaust gas. 12.The method of claim 1 wherein said exhaust gas is directed through saidtube nest at a velocity within the range of 10 to 60 feet per second.13. The method of claim 1 wherein said exhaust gas is passed through ahousing containing said nest of tubes, with a pressure drop within therange of 0.2 to 5 inches of water occurring in said exhaust gas withinsaid housing.
 14. The method of claim 1 which includes the steps ofmonitoring said input temperature of said exhaust gas, and mixing acooler gas with said hot exhaust gas to prevent said input temperaturefrom exceeding a predetermined value.
 15. The method of claim 1 whichincludes the steps of sensing the outlet temperature of said fluid, andvarying the amount of exhaust gas being passed between said tubes ofsaid nest to maintain the temperature of the fluid exiting from saidtube nest at a predetermined value.
 16. The method of claim 1 whereinsaid fluid passed through said tubes comprises water, and said methodincludes the step of supplying water to said tubes from a hot waterstorage means and the step of supplying water from said tubes to saidstorage means.
 17. The method of claim 1 which includes the step ofcontinuously removing water and sulfuric acid which has fallen belowsaid tube nest.
 18. The method of claim 1 which includes the step ofdirecting a spray of liquid onto said nest of tubes.
 19. The method ofclaim 1 wherein said step of passing said exhaust gas comprises forcingsaid exhaust gas through a housing in which said tube nest is located,the inside of said housing being covered with a covering to shield saidhousing from sulfuric acid.
 20. The method of claim 2 which includes thestep of passing said exhaust gas through a heat exchanger before passingsaid exhaust gas to said tube nest, to lower the temperature of saidexhaust gas to a temperature which is below the deformation temperatureof said coverings on said tubes but above the temperature at whichsulfuric acid may condense.
 21. The method of claim 8 which includes thestep of directing said exhaust gas in a generally horizontal directionupon exit of said exhaust gas from said tube nest, thereby to separatecondensation from said exhaust gas.
 22. The method of claim 10 whichincludes the step of directing said exhaust gas in a generally verticaldirection upon exit of said exhaust gas from said tube nest, thereby toseparate condensation from said exhaust gas.
 23. The method ofrecovering heat energy from a hot exhaust gas which initially containswater vapor condensable at a water vapor dewpoint and a corrosiveconstituent condensable at a temperature above said water vapordewpoint, and of recovering latent heat while preventing corrosion fromcondensation of said corrosive constituent, comprising the steps of:passing said exhaust gas in a flow generally perpendicularly to andbetween tubes of a tube nest having upper and lower groups of tubes; andsimultaneously passing a fluid to be heated through successive tubes ofsaid tube nest in counterflow relationship to said gas flow, the amountof said exhaust gas being passed through said tube nest beingestablished in relation to the heat exchange surface area of said tubenest and the input temperature and flow rate of said fluid, and theinitial temperature of said fluid being sufficiently below said watervapor dewpoint so that substantial condensation of water vaporcontinuously occurs, the exterior surfaces of said tube nest beingcovered with coverings which both shield said tubes from condensedcorrosive constituent and provide dropwise condensation of waterdroplets, so that said droplets may drop from tubes of said upper groupthrough said exhaust gas to tubes of said lower group to wash condensedcorrosive constituent from tubes of said lower group, and to dilute thecondensed corrosive constituent.
 24. The method of claim 23 wherein saidhot exhaust gas initially also contains substantial particulate matter,whereby said droplets of water may entrap portions of said particulatematter as they drop through said exhaust gas to said lower ones of saidtubes.
 25. The method of claim 23 wherein said coverings which shieldsaid tubes and provide dropwise condensation of said droplets comprisefluoroplastic coverings having a deformation temperature exceeding theinitial temperature of said hot exhaust gas.
 26. The method of claim 23which includes the step of passing said exhaust gas through a heatexchanger before passing said exhaust gas to said tube nest, to lowerthe temperature of said exhaust gas to a temperature which is below thedeformation temperature of said coverings on said tubes but above thetemperature at which said corrosive constituent will condense.
 27. Themethod of claim 23 wherein said corrosive constituent comprises sulfurtrioxide.
 28. The method of claim 23 wherein said exhaust gas comprisesthe flue gas derived from the combustion of a fuel selected from thegroup consisting of #2 fuel oil, #4 fuel oil, #6 fuel oil and coal. 29.The method of claim 23 which includes the step of introducing ascrubbing liquor into said exhaust gas prior to passing said exhaust gasthrough said tube nest.
 30. The method of claim 23 which includes thesteps of monitoring the initial temperature of said exhaust gas, andmixing a cooler gas with said hot exhaust gas to prevent the temperatureof the gas passed through the tube nest from reaching or exceeding thedeformation temperature of said coverings.
 31. The method of claim 23wherein said step of passing said exhaust gas comprises forcing said gassubstantially downwardly through said tube nest and said step of passingsaid fluid comprises passing said fluid through upwardly successivetubes of said nest, whereby the fluid in said various of said tubes iswarmer than the fluid in said lower ones of said tubes and the fall ofsaid droplets may transfer heat from said various of said tubes to saidlower ones of said tubes.
 32. The method of claim 23 which includes thestep of varying the amount of said exhaust gas being passed through saidtube nest to maintain the temperature of gas exiting from said tube nestat a predetermined value.
 33. The method of claim 23 wherein sufficientcondensation occurs that the latent heat of the condensed water vaporexceeds 5% of the total heat transferred to said fluid.